A tunable laser is a laser system in which the photonic emission can be varied into different wavelengths. Dye lasers have dominated the tunable laser market for many years. In recent years, however, there has been a resurgence of research and development on tunable solid-state laser materials because of their potential advantages over dye lasers. These advantages include long operating life, a longer energy storage time, reliable and efficient flashlamp pumping, Q-Switching, flexible harmonic generation capability, and improved beam quality at high average power levels. The major commercial products that have appeared so far are alexandrite (CR.sup.3+ :BeAl.sub.2 O.sub.4), titanium-sapphire (Ti.sup.3+ :Al.sub.2 O.sub.3, or Ti:Sapphire), and a selection of color center materials that operate at cryogenic temperatures.
In U.S. Pat. No. 4,811,349 two new Cr laser media, Cr:LiCaAlF.sub.6 (Cr:LiCAF) and Cr.sup.3+ : LiSrAlF.sub.6 (Cr:LiSAF) were disclosed. These media have properties which lead to significantly improved tunable laser performance in the near infrared.
The performance of a solid-state laser material at high pulse energies or high average power is determined by its thermal, thermo-optic, thermo-mechanical, and mechanical properties.
Among the problems encountered in high-power operation are mechanical failure due to thermal stress and degradation of beam quality caused by thermally-induced refractive index changes. Good beam quality at high powers requires materials with low thermally-induced refractive index changes. It also requires the use of crystals which are substantially free of certain types of defects, i.e., scattering centers.
The growth of crystals of adequate size and optical quality is probably the most time consuming and uncertain aspect of developing a new laser crystal. Several techniques are known for growing crystals. Crystals of the type described in U.S. Pat. No. 4,811,349 have been pulled from a melt (the Czochralski, or Cz process), and grown by two variants of the Bridgman process, i.e., gradient freeze (GF) and vertical bridgman (VB). By whatever technique used, the crystals develop micron-size scattering centers, or inclusions, which produce scattering losses of approximately 2%/cm. This is due the presence of roughly 10 parts per million by volume. of sub-micron to micron size defects.
As used herein, the term "energy losses" refers to those losses resulting from light being absorbed within the crystals generating heat, or refracted through the walls after colliding with defects and impurities in the crystals.
Defects in crystals develop during the growth process. The nature of the defects depend upon the physical and mechanical properties of the material, the thermal gradients, the growth temperature, and impurity type and concentration.
A discussion of crystal growth and problems associated therewith is set forth in Crystal Growth, A. W. VERE, Plenum Press 1987.
Defects in crystals have been described in terms of twining, grain boundaries, point-defects, dislocations, inclusions, and impurities.
The defects develop due to a number of reasons, e.g., spurious nucleation, too rapid a solidification rate, local fluctuations in temperature at the growth surface, variations in the impurity concentration and the like.
Among the defects produced during crystal growth are so called lattice point-defects. These point-defects include impurity atoms, and native vacancy and interstitial defects.
According to Vere, as a crystalline material is cooled from high temperature, the equilibrium point-defect concentration characteristic of that temperature must be reduced to that of the end point-temperature. Providing the cooling rate is slow enough, the point-defects can migrate to dislocation, grain boundary or surface sinks. If a high rate of cooling is used, the individual point-defects are quenched or frozen into the lattice, giving a high elastic strain. Cooling at intermediate rates or annealing of fast-cool material causes agglomeration of the defects to form small voids or clusters. These defects can collapse to form planar vacancy or interstitial defects bounded by a dislocation loop. These loops, once formed, act as a sink for further condensation and in some materials can grow to several hundred microns in diameter.
In growth techniques involving confinement of the solidified material in a crucible or similar container, expansion of the solid against the rigid wall or uneven contraction also act as sources of elastic strain. In Czochralski growth, although the material is not under stress from a container, the high thermal gradient of the solidified ingot acts as a source of elastic strain, which is again relieved by dislocation generation.
A further source of dislocation generation in cooling crystals is the presence of inclusions or second-phase precipitates. Such particles act as sources of stress in the elastically-strained matrix and give rise to dislocation generation by prismatic punching.
The final density and distribution of dislocations and precipitates in a particular material depends critically on the formation and migration energy of the point-defects, the cooling rate and the cooling temperature interval and the mechanical properties of the lattice and the external stresses.
It would be desirable in the art to provide a method for reducing the extent of defects in crystals and thereby energy losses in such crystals, in order to improve the ultimate output of lasers utilizing the crystals as a source of the laser beam.